drm: include function pointer for vmap

This is the last change in the set of drm_client_modeset
log file. This change is included on top of the dependent changes
to resolve validation issue.
1. Include function pointer for vmap.
2. Remove vmap function in buffer creation.
3. Add file alloc func in open helper.

Change-Id: Id7a7b5d2e987130ce7d7b178a49db19e5eb93b46
Signed-off-by: Shubham Talekar <stalek@codeaurora.org>